1. Field of the Invention
The present invention relates to a laser direct imaging system and method, and more particularly to a laser direct imaging system and method for PCB solder mask exposure.
2. Description of the Related Art
Typically, a solder mask or solder resist is applied to the copper traces of a printed circuit board (PCB) for protection against oxidation and to prevent solder bridges from forming between closely spaced solder pads. Once applied, openings must be made in the solder mask wherever components are soldered, which is accomplished using photolithography.
FIGS. 13-15 depicts a conventional process for forming a patterned solder mask on a PCB. In FIG. 13, there is shown a PCB 9 with copper pads 90 thereon. Next, in FIG. 14, a solder mask material 91 (such as a Liquid PhotoImageable Solder Mask, LPISM) is applied onto the PCB 9, dried in an oven to remove the solvent of the material, and then exposed to a UV light source by using photolithography that removes the un-exposed material, leaving behind the patterned solder mask 91′ as well as the exposed copper pads 90, as shown in FIG. 15. Finally, the solder mask 91′ will be subjected to a thermal process to be completely cured.
As described above, the finished solder mask 91′ has an appearance with high gloss. This results from the UV light source that is typically a high-pressure mercury-vapor lamp (Hg bulbs) and is constructed to emit primarily the ultraviolet radiation within a wavelength range of 320 nm to 410 nm according to the solder mask material.
Other than the photolithography process using the Hg bulbs, a Laser Direct Imaging (LDI) method may also be used for exposure of the solder mask material. In LDI of printed circuit boards, a laser is used to image a pattern directly onto a substrate, completely eliminating the production and use of a traditional photo tool. The most obvious benefits of LDI are the time and costs savings associated with the creation, use, handling and storage of photo tools. However, when using a laser as light source for exposure instead of the usually used Hg bulbs, the exposure wavelength of the laser cannot be completely matched to the spectral sensitivity of the solder mask material designed for the Hg bulbs. The spectral sensitivity of the solder mask material does not abruptly end at a certain wavelength, but smoothly drops to zero over few 10 nm. Thus, as compared to the solder mask formed using the Hg bulbs, the solder mask using the UV laser of a single wavelength, such as 405 nm, has extremely low gloss, which is unacceptable by the consumers. For this manufacturers and PCB fabricators have developed products that offer exposure sensitivity suitable for laser direct imaging.